Inplant morphology,thorns,spines, andprickles, and in generalspinose structures (sometimes calledspinose teeth orspinose apical processes), are hard, rigid extensions or modifications ofleaves,roots,stems, orbuds with sharp, stiff ends, and generally serve the same function: physicallydefending plants against herbivory.
In common language, the terms are used more or less interchangeably, but in botanical terms, thorns are derived fromshoots (so that they may or may not be branched, they may or may not have leaves, and they may or may not arise from a bud),[1][2][3][4] spines are derived fromleaves (either the entire leaf or some part of the leaf that has vascular bundles inside, like thepetiole or astipule),[1][2][3][4] and prickles are derived fromepidermis tissue (so that they can be found anywhere on the plant and do not have vascular bundles inside[4]).[1][2][3]
Leaf margins may also have teeth, and if those teeth are sharp, they are called spinose teeth on a spinoseleaf margin[1][2] (some authors consider them a kind of spine[2]). On a leaf apex, if there is an apical process (generally an extension of the midvein), and if it is especially sharp, stiff, and spine-like, it may be referred to as spinose or as apungent apical process[1] (again, some authors call them a kind of spine[2]). When the leaf epidermis is covered with very long, stifftrichomes (more correctly calledbristles in this case;[1] for some authors a kind of prickle[2]), it may be referred to as ahispid vestiture;[1][2][3] if the trichomes are stinging trichomes, it may be called aurent vestiture.[1]
There can be found also spines or spinose structures derived from roots.[5]
The predominant function of thorns, spines, and prickles is deterringherbivory in a mechanical form. For this reason, they are classified as physical or mechanical defenses, as opposed tochemical defenses.
Not all functions of spines orglochids are limited to defense from physical attacks by herbivores and other animals. In some cases, spines have been shown to shade or insulate the plants that grow them, thereby protecting them from extreme temperatures. For example,saguaro cactus spines shade theapical meristem in summer, and in members of theOpuntioideae, glochids insulate the apical meristem in winter.
Agrawalet al. (2000) found that spines seem to have little effect on specialist pollinators, on which many plants rely in order to reproduce.[6]
Pointing or spinose processes can broadly be divided by the presence of vascular tissue:thorns and spines are derived from shoots and leaves respectively, and have vascular bundles inside, whereasprickles (likerose prickles) do not have vascular bundles inside, so that they can be removed more easily and cleanly than thorns and spines.
Thorns aremodified stems and arise from budsCactus areoles; shoot (yellow), spines (green) andglochids (also spines, green and little)(A) Thorn or spine (B) PrickleA spinose tooth in a leaf marginA spinose apical process
Spines are modifiedleaves,stipules, or parts of leaves, such as extensions of leaf veins. Some authors prefer not to distinguish spines from thorns because, like thorns, and unlike prickles, they commonly containvascular tissue.[7]
Spines are variously described aspetiolar spines (as inFouquieria),leaflet spines (as inPhoenix), orstipular spines (as inEuphorbia), all of which are examples of spines developing from a part of a leaf containing the petiole, midrib, or a secondary vein.[1] The plants of thecactus family are particularly well known for their dense covering of spines. Some cacti have alsoglochids (orglochidia, singular glochidium) – a particular kind of spine of different origin, which are smaller and deciduous with numerous retrose barbs along its length (as found inareoles ofOpuntia).[1]
Prickles are comparable to hairs but can be quite coarse (for example, rose prickles). They are extensions of thecortex andepidermis.[8][9] Technically speaking, many plants commonly thought of as having thorns or spines actually have prickles.Roses, for instance, have prickles.[7] While the position of thorns and spines are known positively to be controlled byphyllotaxis, the positioning of prickles appears to be truly random. If not, then by a phyllotaxis so arcane as to give the appearance of randomness.[citation needed] The largest prickles are found on the trunk and major limbs ofCeiba speciosa which can be 5 cm (2.0 in) long and diameter.
The genusSolanum contains a mixture of species with and without prickles.Phylogenetic analysis in 2024 concluded that members of a gene family known for involvement incytokinin biosynthesis were involved in prickle production.[10]
Other similar structures are spinose teeth, spinose apical processes, and trichomes.Trichomes, in particular, are distinct from thorns, spines, and prickles in that they are much smaller (often microscopic) outgrowths of epidermal tissue, and they are less rigid and more hair-like in appearance; they typically consist of just a few cells of the outermost layer of epidermis, whereas prickles may include cortex tissue. Trichomes are often effective defenses against small insect herbivores; thorns, spines, and prickles are usually only effective against larger herbivores like birds and mammals.
Spinescent is a term describing plants that bear any sharp structures that deter herbivory. It also can refer to the state of tending to be or become spiny in some sense or degree, as in: "... the division of the African acacias on the basis of spinescent stipules versus non-spinescent stipules..."[11]
"Root spines" on the trunk of aCryosophila species.
There are also spines derived from roots, like the ones on the trunk of the "root spine palms" (Cryosophila spp.). The trunk roots ofCryosophila guagara grow downwards to a length of 6–12 cm, then stop growing and transform into a spine.[5] The anatomy of crown roots on this species (roots among the bases of the living fronds) also alters during their life.[5] They initially grow upwards and then turn down and finally they, too, become spinous.[5] Lateral roots on these two types of roots, as well as those on the stilt roots on this species, also become spinous.[5] Some authors believe that some of these short spiny laterals have a ventilating function so they are 'pneumorhizae'.[5] Short spiny laterals that may have a ventilating function may also be found on roots ofIriartea exorrhiza.[5]
There are also spines that function as pneumorhizae on the palmEuterpe oleracea.[5] InCryosophila nana (formerlyAcanthorhiza aculeata), there are spiny roots; some authors prefer to term these "root spines" if the length of the root is less than 10x the thickness and "spine roots" if the length is more than 10x the thickness.[5] Adventitious spiny roots have also been described on the trunks of dicotyledonous trees from tropical Africa (e.g. Euphorbiaceae, as inMacaranga barteri,Bridelia micrantha andB. pubescens; Ixonanthaceae, Sterculiaceae), and may also be found protecting perennating organs such as tubers and corms (e.g.Dioscorea prehensilis -Dioscoreaceae- andMoraea spp. -Iridaceae- respectively).[5] Short root spines cover the tuberous base of the epiphytic ant-plantMyrmecodia tuberosa (Rubiaceae), these probably give protection to ants which inhabit chambers within the tuber as they wander over the plant's surface. (Jackson 1986[5] and references therein). In many respects, the pattern of spine formation is similar to that which occurs in the development of thorns from lateral shoots. (Jackson 1986[5] and references therein).
It has been proposed that thorny structures may have first evolved as adefense mechanism in plants growing in sandy environments that provided inadequate resources for fast regeneration of damage.[12][13]
Plants bearing thorns, spines, or prickles are often used as a defense againstburglary, being strategically planted below windows or around the entire perimeter of a property.[17] They also have been used to protect crops and livestock against marauding animals. Examples includehawthorn hedges in Europe,agaves orocotillos in the Americas and in other countries where they have been introduced,Osage orange in the prairie states of the US, andSansevieria in Africa.[18][page needed]
^Agrawal, Anurag A.; Rudgers, Jennifer A.; Botsford, Louis W.; Cutler, David; Gorin, Jessica B.; Lundquist, Carolyn J.; Spitzer, Brian W.; Swann, Alisa L. (2000). "Benefits and Constraints on Plant Defense against Herbivores: Spines Influence the Legitimate and Illegitimate Flower Visitors of Yellow Star Thistle, Centaurea solstitialis L. (Asteraceae)".The Southwestern Naturalist.45 (1):1–5.Bibcode:2000SWNat..45....1A.doi:10.2307/3672545.ISSN0038-4909.JSTOR3672545.
^abBell, A.D. 1997.Plant form: an illustrated guide to flowering plant morphology. Oxford University Press, Oxford, U.K.preview in google books
^Van Wyk, Braam (2007).How to Identify Trees in Southern Africa (illustrated ed.). Struik. p. 184.ISBN978-1-77007-240-4.
^Ross, J. H. "A conspectus of the African Acacia species." Series: Memoirs of the Botanical Survey of South Africa, No. 44 Botanical Research Institute, Dept. of Agricultural Technical Services, Pretoria, 1979
^Steve Brill, Evelyn Dean,Identifying and Harvesting Edible and Medicinal Plants (1994), p. 17.
^August Weismann, John Arthur Thomson, Margaret R. Thomson,The Evolution Theory (1904), p. 124.
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